Multilayer interference pigments

ABSTRACT

Multilayer interference pigment consisting of a transparent carrier material coated with alternating layers of metal oxides of low and high refractive index, the difference in the refractive indices being at least 0.1, which is obtainable by alternate coating of the transparent carrier material with a metal oxide of high refractive index and with a metal oxide of low refractive index in a wet process by hydrolysis of the corresponding water-soluble metal compounds, separation, drying and, if desired, calcination of the resulting pigment.

[0001] The invention relates to multilayer interference pigmentsconsisting of a transparent carrier material coated with alternatinglayers of a metal oxide of low refractive index and a metal oxide ofhigh refractive index.

[0002] Multilayer pigments of low transparency and with a similar layerstructure are known. The metal oxide layers are prepared either in a wetprocess, by precipitating the metal oxide hydrates from a metal saltsolution onto a carrier material, or by vapour deposition or sputteringin a vacuum. In general, the vapour deposition processes are too complexand costly for mass production of pigments. Thus U.S. Pat. No. 4,434,010describes a multilayer interference pigment consisting of a centrallayer of a reflecting material (aluminium) and alternating layers of twotransparent, dielectric materials of high and low refractive index, forexample titanium dioxide and silicon dioxide, either side of the centralaluminium layer. This pigment is employed for the printing ofsecurities.

[0003] JP H7-759 (Kokoku) describes a multilayer interference pigmentwith a metallic lustre. It consists of a substrate coated withalternating layers of titanium dioxide and silicon dioxide. Thesubstrate is formed from flakes of aluminium, gold or silver or fromplatelets of mica and glass which are coated with metals. Accordingly,it is a typical metallic pigment. This pigment is of high opacity. Forapplications where a high level of transparency of the pigmentedmaterial is required, as for example for agricultural films, the pigmentis unsuitable. Furthermore, it has the disadvantage that the deptheffect typical of interference pigments is not produced since, owing tothe high reflection of light at the metal layer which forms the core,pigment particles lying deeper in the application medium are unable tocontribute to the optical appearance. The interference effect thereforeremains limited to the layers located on the metal layer.

[0004] The object of the invention is to provide an essentiallytransparent interference pigment having strong interference coloursand/or a strong angular dependency of the interference colours.Furthermore, the object of the invention is to provide pigments havingspecific spectral characteristics in the visible region and in theinfrared region.

[0005] This object is achieved in accordance with the invention by amultilayer interference pigment consisting of a transparent carriermaterial coated with alternating layers of metal oxides of low and highrefractive index, the difference in the refractive indices being atleast 0.1, which is obtainable by alternate coating of the transparentcarrier material with a metal oxide of high refractive index and with ametal oxide of low refractive index in a wet process by hydrolysis ofthe corresponding water-soluble metal compounds, separation, drying and,if desired, calcination of the resulting pigment.

[0006] The transparent carrier material is mica, a differentphyllosilicate, glass flakes, PbCO₃×Pb(OH)₂ and BiOCl in platelet form,or plateletlike silicon dioxide prepared by the process described in WO93/08237.

[0007] The metal oxide of high refractive index can be an oxide ormixtures of oxides with or without absorbing properties, such as TiO₂,ZrO₂, Fe₂O₃, Fe₃O₄, Cr₂O₃, or ZnO, for example, or a compound of highrefractive index, for example iron titanates, iron oxide hydrates andtitanium suboxides, or mixtures and/or mixed phases of these compoundswith one another or with other metal oxides.

[0008] The metal oxide of low refractive index is SiO₂, Al₂O₃, Al₂OH,B₂O₃ or a mixture thereof and can likewise have absorbing ornonabsorbing properties. If desired, the oxide layer of low refractiveindex may include alkali metal oxides and alkaline earth metal oxides asconstituents.

[0009] This object is additionally achieved in accordance with theinvention by a process for the preparation of the novel pigments, inwhich the transparent carrier material is suspended in water and coatedin alternation with a metal oxide hydrate of high refractive index andwith a metal oxide hydrate of low refractive index by addition andhydrolysis of the corresponding water-soluble metal compounds, the pHnecessary for the precipitation of the respective metal oxide hydratebeing established and held constant by simultaneous addition of acid orbase, and then the coated carrier material is separated off from theaqueous suspension, dried and, if desired, calcined.

[0010] The invention additionally relates to the use of the novelpigments for pigmenting paints, printing inks, plastics, glazes forceramics and glass, cosmetics and, in particular, for the production ofagricultural films.

[0011] For this purpose they can be employed as mixtures with customarycommercial pigments, for example inorganic and organic absorptionpigments, metallic pigments and LCP pigments.

[0012] The thickness of the layers of the metal oxides of high and lowrefractive index is critical for the optical properties of the pigment.Since a product with powerful interference colours is desired, thethicknesses of the layers must be adjusted relative to one another. If nis the refractive index of a layer and d its thickness, the interferencecolour of a thin layer is the product of n and d, i.e. the opticalthickness. The colours of such a film, as produced with normal incidenceof light in reflected light, result from an intensification of the lightof wavelength λ=(4/2N−1) nd and by attenuation of light of wavelengthλ=(2/N)·nd, where N is a positive integer. The variation in colour whichtakes place as the thickness of the film increases results from theintensification or attenuation of particular wavelengths of the light byinterference. For example, a 115 nm film of titanium dioxide ofrefractive index 1.94 has an optical thickness of 115×1.94=223 nm, andlight of wavelength 2×223 nm=446 nm (blue) is attenuated in the courseof reflection, with the result that the reflected light is yellow. Inthe case of multilayer pigments, the interference colour is determinedby the intensification of specific wavelengths and, if two or morelayers in a multilayer pigment possess the same optical thickness, thecolour of the reflected light becomes more intense and full as thenumber of layers increases. Moreover, by a suitable choice of the layerthicknesses it is possible to achieve a particularly marked variation ofcolour in dependency on the viewing angle. A pronounced colour flopdevelops, which may be desirable for the pigments according to theinvention. The thickness of the individual metal oxide layers,independently of their refractive index, is therefore from 20 to 500 nm,preferably from 50 to 300 nm.

[0013] The number and thickness of the layers is dependent on thedesired effect and on the substrate used. On mica, the desired effectsare achieved if the 3-coat system TiO₂/SiO₂/TiO₂ is built up and if thethicknesses of the individual layers are matched optically to oneanother. When using optically relatively thin TiO₂ and SiO₂ layers(layer thickness <100 nm) it is possible, for example, to producepigments with a blue interference colour which, with a substantiallysmaller TiO₂ content, are stronger in colour and more transparent thanpure TiO₂-mica pigments. The saving in terms of TiO₂ is up to 50% byweight.

[0014] By means of the precipitation of thick SiO₂ layers (layerthickness >100 nm), pigments having a strongly pronounced angulardependency of the interference colour are obtained.

[0015] By precipitating further TiO₂ and SiO₂ layers it is also possibleto obtain 5-layer and higher systems, but then the number of layers islimited by the economics of the pigment.

[0016] However, if SiO₂ platelets of uniform layer thickness are used assubstrate instead of mica, then further, particularly well-definedinterference effects can be achieved.

[0017] In this case, covering the substrate with, for example, 3 layersof the abovementioned structure produces an interference systemcomprising 7 thin layers of sharply defined thicknesses. The reflectionor transmission spectrum of such a pigment exhibits finer and moreprecisely matchable structures than the spectrum of a correspondingpigment based on a substrate with a broad distribution of thickness,such as mica.

[0018] Even with extremely thin TiO₂ layers (layer thickness <50 nm),these pigments exhibit powerful interference colours. The angulardependency of the interference colour is also particularly pronounced.This extreme colour flop is not observed with conventional metaloxide-mica pigments.

[0019] The SiO₂ platelets are prepared, for example, in accordance withinternational application WO 93/08237 on a continuous belt bysolidification and hydrolysis of an alkali metal silicate solution.

[0020] The metal oxide layers are preferably applied by a wet-chemicalprocess which may be one of those wet-chemical coating processesdeveloped for the preparation of pearl lustre pigments; processes ofthis kind are described, for example, in DE 14 67 468, DE 19 59 988, DE20 09 566, DE 22 14 545, DE 22 15 191, DE 22 44 298, DE 23 13 331, DE 2522 572, DE 31 37 808, DE 31 37 809, DE 31 51 343, DE 31 51 354, DE 31 51355, DE 32 11 602 and DE 32 35 017 or else in further patent documentsand other publications.

[0021] For the coating, the substrate particles are suspended in water,and one or more hydrolysable metal salts are added at a pH which issuitable for the hydrolysis and is chosen so that the metal oxidesand/or metal oxide hydrates are precipitated directly onto the particleswithout any instances of secondary precipitation. The pH is usually keptconstant by simultaneous metered addition of a base. The pigments arethen separated off, washed and dried and, if desired, calcined, it beingpossible to optimize the calcining temperature with respect to theparticular coating present. If desired, pigments to which individualcoatings have been applied can be separated off, dried and, if desired,calcined before being resuspended in order to apply the further layersby precipitation.

[0022] Furthermore, coating can also be carried out in a fluidized-bedreactor by gas phase coating, in which context it is impossible, forexample, to employ correspondingly the techniques proposed for thepreparation of pearl lustre pigments in EP 0 045 851 and EP 0 106 235.

[0023] The metal oxide of high refractive index used is preferablytitanium dioxide, and the metal oxide of low refractive index preferablyused is silicon dioxide.

[0024] For the application of the titanium dioxide layers the processdescribed in U.S. Pat. No. 3,553,001 is preferred.

[0025] An aqueous titanium salt solution is added slowly to asuspension, heated to about 50-100° C., in particular 70-8° C., of thematerial to be coated, and a substantially constant pH of about 0.5-5,in particular about 1.5-2.5, is maintained by simultaneous meteredaddition of a base, for example aqueous ammonia solution or aqueousalkali metal hydroxide solution. As soon as the desired layer thicknessof the TiO₂ precipitation has been reached, the addition of the titaniumsalt solution and of the base is stopped.

[0026] This process, also termed the titration process, is notable forthe fact that it avoids an excess of titanium salt. This is achieved bysupplying to the hydrolysis only that quantity per unit time which isnecessary for uniform coating with the hydrated TiO₂ and which can bereceived per unit time by the available surface area of the particles tobe coated. There is therefore no production of hydrated titanium dioxideparticles not precipitated on the surface to be coated.

[0027] For the application of the silicon dioxide layers, the followingprocess can be employed: a sodium silicate solution is metered into asuspension, heated to about 50-100° C., in particular 70-80° C., of thematerial to be coated. The pH is held constant at 4-10, preferably at6.5-8.5, by simultaneous addition of 10% hydrochloric acid. Stirring iscarried out for 30 minutes following addition of the silicate solution.

[0028] It is also possible to alter the powder colour of the pigment byapplying further layers, for example coloured metal oxides or Prussianblue, transition metal compounds, such as compounds of Fe, Cu, Ni, Co orCr, for example, or organic compounds such as dyes or colour lakes.

[0029] The wet-chemical production of 2 or more interference layers ofdifferent refractive index with precisely defined thicknesses on finelydivided plateletlike substrates in an aqueous medium using purelyinorganic starting materials has not been disclosed hitherto.

[0030] It is additionally possible to subject the finished pigment to anaftercoating or aftertreatment process which further increases thestability to light, weather and chemicals, or which facilitates thehandling of the pigment, especially its incorporation into differentmedia. Suitable aftercoating and aftertreatment processes are thosedescribed, for example, in DE-C 22 15 191, DE-A 31 51 354, DE-A 32 35017 or DE-A 33 34 598.

[0031] The substances additionally applied make up only about 0.1-5% byweight, preferably about 0.5-3′ by weight, of the overall pigment.

[0032] In addition, the novel pigment can also be coated with thermallyadhering inorganic or organic colorants of low solubility. Preference isgiven to the use of colour lakes and, in particular, aluminium colourlakes. For this purpose a layer of aluminium hydroxide is applied byprecipitation and in a second step is laked with a colour lake. Theprocess is described in more detail in DE 24 29 762 and DE 29 28 287.

[0033] Preference is also given to an additional coating with complexsalt pigments, especially cyanoferrate complexes, for example Prussianblue and Turnbull's blue, as is described in EP 0 141 173 and DE 23 13332.

[0034] The novel pigment can also be coated with organic dyes and, inparticular, with phthalocyanine or metal phthalocyanine and/orindanthrene dyes in accordance with DE 40 09 567. To this end asuspension of the pigment in a solution of the dye is prepared and thissolution is then brought together with a solvent in which the dye is oflow or zero solubility.

[0035] Furthermore, metal chalcogenides or metal chalcogenide hydratesand carbon black can also be employed for an additional coating.

[0036] The pigment can be used in a conventional manner for pigmentingpaints, printing inks, plastics, cosmetics and glazes for ceramics andglass. It is preferably used for pigmenting agricultural films.

[0037] Agricultural films are often treated with pigments in order tokeep out the infrared radiation of the sun and thus to preventoverheating in, for example, a greenhouse.

[0038] Almost all of the pigments used to date in agricultural-films arecolour pigments. Therefore, they absorb or reflect a substantialproportion of the visible light which, however, is required by theplants living under the film for their growth. As a consequence, thepigments used to date in agricultural films have an adverse effect onthe growth behaviour of the plants.

[0039] It is therefore an object of the present invention to provide aninterference pigment having high transmissibility in the visible regionof light and high reflectivity in the NIR region. The properties of suchpigments can also be adjusted so that they have other or additionalfunctions, for example to influence the morphogenis of plants in acontrolled manner.

[0040] The examples which follow are intended to illustrate theinvention in more detail without limiting it.

EXAMPLE 1

[0041] 3-Layer System with Thin SiO₂ Layer

[0042] 1) First TiO₂ Layer:

[0043] 150 g of mica (particle size 10-40 μm) are suspended in 2 l ofdeionized water and the suspension is heated to 75° C. 175 ml of anaqueous TiC₄ solution (400 g of TiCl₄) is metered into this suspensionover the course of 60 minutes. Throughout the addition the pH is heldconstant at 2.2 with 32% NaOH solution. When addition is over, stirringis carried out at 75° C. for 30 minutes in order to complete theprecipitation.

[0044] 2) SiO₂ layer:

[0045] The pH of the suspension is raised to 7.5 with NaOH solution, and250 ml of a sodium silicate solution (125 g of SiO_(2/1)) are metered inat 75° C. over the course of 90 minutes. During this time, the pH isheld constant with 10% hydrochloric acid. When addition is over,stirring is carried out at 75° C. for 0.30 minutes again in order tocomplete the precipitation.

[0046] 3) Second TiO₂ Layer:

[0047] The pH is reduced again to 2.2 with 10% hydrochloric acid, andTiO₂ is again applied by precipitation from a further 175 ml of TiCl₄solution, as described in step 1).

[0048] The mixture is then cooled to room temperature, and the pigmentobtained is filtered off, washed salt-free with deionized water anddried at 1100° C.

[0049] The pigment is then calcined at 850° C. for 30 minutes.

[0050] The pigment thus obtained is notable for the more intense blueinterference colour and higher transparency than comparable pureTiO₂-mica pigments.

EXAMPLE 2

[0051] 3-Layer System with Thick SiO₂ Layer

[0052] 1) First TiO₂ Layer:

[0053] 150 g of mica (particle size 10-40 μm) are suspended in 2 l ofdeionized water and the suspension is heated to 750° C. 300 ml of anaqueous TiCl₄ solution (400 g. of TiCl_(4/1)) is metered into thissuspension over the course of 100 minutes. Throughout the addition thepH is held constant at 2.2 with 32% NaOH solution. When addition isover, stirring is carried out at 750° C. for 30 minutes in order tocomplete the precipitation.

[0054] 2) SiO₂ Layer:

[0055] The pH of the suspension is raised to 7.5 with NaOH solution, and1350 ml of a sodium silicate solution (125 g SiO₂/l) are metered in at75° C. over the course of 7.5 hours. During this time, the pH is heldconstant with 10% hydrochloric acid. When addition is over, stirring iscarried out at 750° C. for 30 minutes again in order to complete theprecipitation.

[0056] 3) Second TiO₂ Layer:

[0057] The pH is reduced again to 2.2, and TiO₂ is again applied byprecipitation from a further 250 ml of TiCl₄ solution, as described instep 1).

[0058] The mixture is then cooled to room temperature, and the pigmentobtained is filtered off, washed salt-free with deionized water anddried at 110° C. The pigment is then calcined at 850° C. for 30 minutes.

[0059] The pigment thus obtained when viewed straight on exhibits anintense blue-green interference colour which changes through violet intored when the pigment is tilted.

EXAMPLE 3

[0060] 3-Layer System with Fe₂O₃ Layer

[0061] This example describes a layer structure in which the 3rd layeris not TiO₂ again but instead Fe₂O₃.

[0062] 1) TiO₂ Layer:

[0063] As described in Example 1.

[0064] 2) SiO₂ Layer:

[0065] As described in Example 1.

[0066] 3). Fe₂O₃ Layer:

[0067] The pH of the suspension of the mica coated with TiO₂ and SiO₂ isadjusted to 3.0 using 10% hydrochloric acid. Then 1750 ml of an aqueousFeCl₃ solution (35 g Fe/l) are metered in at 75° C. over the course of 5hours while maintaining the pH at a constant level by simultaneousaddition of 32 ? NaOH. Stirring is then carried out at 75° C. for 45minutes in order to complete the precipitation.

[0068] The mixture is then cooled to room temperature, and the red-brownpigment obtained is filtered off, washed salt-free with deionized waterand dried at 110° C. The pigment is then calcined at 850° C. for 30minutes. An orange-brown pearl lustre pigment with a copper likeinterference colour is obtained.

EXAMPLE 4

[0069] Multilayer System Comprising 5 Alternating TiO₂ and SiO₂ Layers

[0070] As described in Example 1, mica is coated with TiO₂, SiO₂ andTiO₂. The another SiO₂ and a final TiO₂ layer are applied. Working up isas described above.

[0071] The pigment obtained has a clearer blue interference colour andhigher transparency than that of Example 1.

EXAMPLE 5

[0072] 3-Layer System with High Transparency in the Visible Region andHigh Reflection in the Near Infrared Region

[0073] 100 g of mica (particle size 10-60 μm) are suspended in 2 l ofdeionized water and the suspension is heated to 80° C. with vigorousstirring. A solution of 3 g of SnCl₄×5H₂O and 10 ml of hydrochloric acid(37%) in 90 ml of deionized water is metered into this mixture at a pHof 2.0 and at a rate of 4 ml/min. Then, at a pH of 1.8, a quantity of481 ml of TiCl₄ solution (400 g TiCl_(4/1)) is metered in at a rate of 2ml/min. The pH is then adjusted to 7.5 with sodium hydroxide solution(32%) and, at this pH, a solution of 230 ml of sodium silicate (fromMerck; Order No. 5621) in 314 ml of deionized water is metered in at arate of 2 ml/min. The pH during this procedure is kept constant withhydrochloric acid (10%). Then, at a pH of 2.0, a solution of 3 g ofSnCl₄×5H₂O and 10 ml. of hydrochloric acid (32%) in 90 ml of deionizedwater is metered in at a rate of 4 ml/min. Then, at a pH of 1.8, 481 mlof TiCl₄ solution (400 g TiCl_(4/1)) are metered in at a rate of 2ml/min.

[0074] The pH is held constant, in each case with NaOH solution (32%),during the addition of SnCl₄×5H₂O solutions and TiCl₄ solutions.

[0075] For working up, the pigment is filtered off, washed with 20 l ofdeionized water, dried at 1100° C. and calcined at 8500° C. for 30minutes.

[0076] This pigment was used to produce a paint film whose transmissionspectrum is reproduced in FIG. 1. The pigment is notable for a very goodtransparency in the visible region of light and a very high reflectionin the near infrared region, properties which cannot be achieved withconventional interference pigments. Consequently, this pigment isparticularly suitable for use in agricultural films.

EXAMPLE 6

[0077] Pigment with High Angular Dependency of the Colour

[0078] 100 g of a TiO₂-mica pigment (particle size 10-60 μm, 35% TiO₂)are suspended in 2 l of deionized water and the suspension is heated to80° C. with vigorous stirring. The pH is then adjusted to 7.5 withsodium hydroxide solution (32%) and, at this pH, a solution of 296 ml ofsodium silicate (from Merck; Order No. 5621) in 300 ml of deionizedwater is metered in at a rate of 2 ml/min. During this addition, the pHis held constant with hydrochloric acid (10%). Then, at a pH of 2.0, asolution of 3 g of SnCl₄×5H₂O and 10 ml of hydrochloric acid (37%) in 90ml of deionized water is metered into this mixture at a rate of 4ml/min. Subsequently, at a pH of 1.8, a quantity of 238 ml of TiCl₄solution (400 g TiCl₄/l) is metered in at a rate of 2 ml/min.

[0079] The pH is kept constant, in each case using NaOH solution (32%),during the addition of the SnCl₄×5H₂O solution and the TiCl₄ solution.

[0080] For working up the pigment is filtered off, washed with 20 l ofdeionized water, dried at 110° C. and calcined at 850° C. for 30minutes.

[0081] The calcined pigment is stirred into a clearcoat (concentration1.7%) which is applied to a black/white card.

[0082] The interference colour of the pigment is very much in evidence,especially on the black card. When the card is tilted from a steep to aflat viewing angle, the reflected colour changes from intense blue tointense violet. The reflection spectrum of the black card was measuredunder various steep (800/1000) and flat (250/1550) observation angles.The reflection curves are reproduced in FIG. 2.

EXAMPLE 7

[0083] 3-Layer System with SiO₂ Platelets as Carrier Material

[0084] 1) First TiO₂ Layer:

[0085] 100 g of SiO₂ platelets (particle size 20-70 μm) are suspended in10.5 l of deionized water and the suspension is heated to 75° C. 160 mlof an aqueous TiCl₄ solution (400 g of TiCl₄/l) is metered into thissuspension over the course of 90-minutes. Throughout the addition the pHis held constant at 2.2 with 32% NaOH solution. When addition is over,stirring is carried out at 750° C. for 30 minutes in order to completethe precipitation.

[0086] 2) SiO₂ Layer:

[0087] The pH of the suspension is raised to 7.5 with NaOH solution, and720 ml of a sodium silicate solution (125 g of SiO_(2/1)) are metered inat 750° C. over the course of 3.5 h. During this time, the pH is heldconstant with 10% hydrochloric acid. When addition is over, stirring iscarried out at 700° C. for 30 minutes again in order to complete theprecipitation.

[0088] 3) Second TiO₂ Layer:

[0089] The pH is reduced again to 2.2 and TiO₂ is again applied byprecipitation from a further 235 ml of TiCl₄ solution, as described instep 1).

[0090] The mixture is then cooled to room temperature, and the pigmentobtained is filtered off, washed salt-free with deionized water anddried at 1100° C. The pigment is then calcined at 850° C. for 30minutes. The pigment thus obtained, when viewed straight on, exhibits abrilliant yellow-green interference colour which changes throughblue-green to dark violet when the pigment is tilted.

1. Multilayer interference pigment consisting of a transparent carriermaterial coated with alternating layers of metal oxides of low and highrefractive index, the difference in the refractive indices being atleast 0.1, which is obtainable by alternate coating of the transparentcarrier material with a metal oxide of high refractive index and with ametal oxide of low refractive index in a wet process by hydrolysis ofthe corresponding water-soluble metal compounds, separation, drying and,if desired, calcination of the resulting pigment.
 2. Interferencepigment according to claim 1, characterized in that the transparentcarrier material is mica, a different phyllosilicate, glass flakes,PbCO₃×Pb(OH)₂, BiOCl or plateletlike SiO₂.
 3. Interference pigmentaccording to claims 1 and 2, characterized in that the oxide of highrefractive index is TiO₂, ZrO₂, Fe₂O₃, Fe₃O₄, Cr₂O₃ ZnO or a mixture ofthese oxides or an iron titanate, iron oxide hydrate, a titaniumsuboxide or a mixture or mixed phase of these compounds.
 4. Interferencepigment according to at least one of claims 1 to 3, characterized inthat the metal oxide of low refractive index is SiO₂, Al₂O₃, Al₂OH, B₂O₃or a mixture thereof, it being possible if desired for alkali metaloxides or alkaline earth metal oxides to be present as additionalconstituents.
 5. Process for the preparation of the interference pigmentaccording to claims 1 to 4, characterized in that the transparentcarrier material is suspended in water and coated in alternation with ametal oxide hydrate of high refractive index and with a metal oxidehydrate of low refractive index by addition and hydrolysis of thecorresponding water-soluble metal compounds, the pH necessary for theprecipitation of the respective metal oxide hydrate being establishedand held constant by simultaneous addition of acid or base, and then thecoated carrier material is separated off from the aqueous suspension,dried and, if desired, calcined.
 6. Process according to claim 5,characterized in that the transparent carrier material employed is mica,a different phyllosilicate, PbCO₃×Pb(OH)₂₁ BiOCl or plateletlike SiO₂.7. Process according to claims 5 and 6, characterized in that the metaloxide of high refractive index is TiO₂, ZrO₂, Fe₂O₃, Fe₃O₄, Cr₂O₃ orZnO.
 8. Process according to at least one of claims 5 to 7,characterized in that the metal oxide of low refractive index is SiO₂,Al₂O₃, AlOH, B₂O₃ or a mixture thereof, it being possible if desired foralkali metal oxides or alkaline earth metal oxides to be present asadditional constituents.
 9. Process according to at least one of claims5 to 8, characterized in that the metal oxides are applied, afterintermediate drying of the material to be coated, in a fluidized-bedreactor by CVD.
 10. Use of the pigments according to claims 1 to 4 forpigmenting paints, printing inks, plastics, cosmetics, glazes forceramics and glasses.
 11. Use according to claim 10, characterized inthat the pigments are employed as mixtures with customary commercialpigments.
 12. Use of the pigments according to claims 1 to 4, for thelaser marking of plastics.
 13. Paints, printing inks, plastics,cosmetics, ceramics and glasses pigmented with a pigment according toclaims 1 to
 4. 14. Laser-markable plastics comprising pigments accordingto claims 1 to 4.